More than a decade of debate in vision theory has centered on whether Ca2+ or cyclic GMP is the messenger between bleached rhodopsin and the plasmalemma of the vertebrate retinal outer segment (ROS). While the scales have recently been tipped in favor of cGMP, Ca2+ is far from dead, and meanwhile a third candidate has come upon the scene - myo-inositol 1,4,5-trisphosphate (IP3). The proposed work is directed at exploring regulation of the anabolic and catabolic reactions associated with the appearance and disappearance of this candidate messenger, and especially its relationship to light and to the other candidates. We intend to (1) determine the regulatory mechanisms of anabolic and catabolic phosphoinositide metabolism in bovine ROS, particularly as related to light, Ca2+ and other metal cations, GTP-binding protein, GTP, cGMP and other nucleotides; (2) isolate key soluble, peripheral or membrane-bound components of ROS phosphoinositide metabolism; (3) reconstitute these components in functional assays confirming essential regulatory mechanisms; (4) make antibodies to purified components, and localize the components in the ROS by immunocytochemistry.